Donald Trump, the persistent front-runner in the crowded field of Republican presidential hopefuls, sought to dampen the rising candidacy of Dr. Ben Carson on Sunday, questioning whether the retired neurosurgeon has the energy and deal-making skill to lead the nation.

In several national polls, Trump, a billionaire businessman, and Carson lead the pack of GOP presidential candidates. Both are appealing to voters because of their outsider status.

Trump has castigated anyone who has even slightly criticized his campaign — and has made Carson his most recent target after he seemed to question Trump’s religious faith last week.

“I’m a deal maker. I’ll make great deals for this country,” said Trump, speaking on CBS’ “Face the Nation.” “Ben can’t do that. Ben’s a doctor, and he’s not a deal maker and I’ll make great deals for our country, which is very important.”

While also a guest on “Face the Nation,” Carson dismissed any criticism that he would not be able to lead the country. He cited his career outside of the operating room in which he’s served on several corporate boards, including Kellogg Co. and Costco Wholesale Corp.

“It’s ridiculous to think that the only thing that I can do is neurosurgery. I find it quite humorous when people say, ‘He’s an idiot savant. He only knows neurosurgery,’ ” he said. “Just the fact that they would say something like that, they don’t know what it takes to become a neurosurgeon. That’s pretty idiotic itself.”

The sparring comes as Carson, the former head of pediatric neurosurgery at Johns Hopkins Hospital, rises in the polls and has, at times, looked to distance himself from the bombast of Trump’s candidacy.

At a campaign event in Southern California last week, Carson said what separates him from Trump is he does not deny a “faith in God.” Carson offered an apology days later, saying he did not mean to offend Trump.

When asked whether Trump is humble enough to be president, Carson noted only it “will be a decision that the voters will make.”

While campaigning in Iowa on Saturday, Trump cast doubt that Carson has the energy to lead the country. In an interview Sunday on ABC’s “This Week,” Carson disagreed.

“It doesn’t bother me, because I recognize that I have plenty of energy,” Carson said. “You know, operating on people for 10, 12, sometimes for greater than 20 hours at a time, making critical decisions after many hours of intense work, having unexpected situations come up. You don’t have to be loud to be energetic.”

Nationally, the two men are polling at the top of the field. In California, a University of Southern California Dornsife/Los Angeles Times poll released Sunday found 24 percent of Republicans support Trump and 18 percent support Carson.

The poll came just days before the top 11 candidates, who include former Florida Gov. Jeb Bush and current Florida Sen. Marco Rubio, travel to Simi Valley, Calif., on Wednesday for the Reagan Presidential Library debate.